Autofocus devices for still projectors using controllable measuring radiation sources are known.
EP-PS 0131 794 discloses an autofocus device for still projectors in which an infrared light emitting diode is used as a measuring radiation source which is operated by means of the half-wave voltage of a power rectifier and whose emission is controlled in response to the signal obtained from a dual photoreceiver. The device also includes a differential amplifier which amplifies and rectifies only the alternating portion of the signals received from the dual photoreceiver. An additional amplifier amplifies the rectified signal to activate a d.c. motor of a focussing drive.
It is the object of the present invention to provide an autofocus device of the generic type which effects fast and reliable focussing of a slide and which, in particular, is insensitive to interference signals and component tolerances and has a short reaction time and high positioning accuracy. Moreover, it should be guaranteed that slides which have different reflection factors are also quickly and reliably focussed.